The pipetting of fluids, for example, in laboratory environments, has been accomplished by a variety of methods. Traditional mouth pipetting techniques have been abandoned in favor of a variety of mechanical pipetting devices. Various mechanical devices such as pipet guns are known which use vacuum and air pressure for aspirating and discharging fluids, for example, drawing liquids into and expelling liquids from pipets. A typical pipet gun comprises a hand-held unit in communication with a laboratory pipet at one end and connected to either a remote or local air pressure source at the other end. Trigger-operated valves located within the pipet gun regulate the flow of air through the gun and to the pipet to control either the intake or expulsion of liquid through the pipet. The operator regulates air flow to the pipet by depressing either a positive pressure trigger or a negative pressure trigger on the pipet gun. The magnitude of the pressure is typically predetermined and controlled by the valves located within the pipet gun housing. Some pipets guns are provided with a universal nose piece attachment for cooperating and communicating with pipets of various lengths and diameters.
A portable pipet gun is shown and described in U.S. Pat. No. 5,214,968. The pipet gun includes a pair of trigger-operated valves. Selective operation of the valves triggers a portable pump which establishes a vacuum to draw liquid into the pipet or creates pressure to dispense liquid from the pipet. This pipet gun, however, does not provide for any variation in speed.
It is desired for practical use of such pipet guns to provide variable flow rates, so as to control the flow rate of fluid into and out of a pipet. For instance, while a low flow rate is preferred for precise metering of liquid samples in small pipets, a low flow rate is inefficient for larger pipets. Pipet guns which are capable of variable control in the rate of liquid flow through the pipet are known in the art. For example, it is known to variably control the air pressure at the pressure source by incorporating a speed control on the pressure pump. When the motor speed of such a pump is increased, however, the pump has a tendency to jerk irregularly, causing a temporary irregular flow rate through the pipet, thereby creating the potential for error in accuracy in pipetting.
U.S. Pat. No. 3,963,061 discloses a conventional pipet gun for convenient pipetting of liquids which includes an adjustable valve for continuously variably controlling the pressure applied to the pipet from a constant pressure source. The pipet gun includes a generally pistol-grip-shaped handle portion and a pipet-supporting portion. The pipet gun is attached to a vacuum source and an air pressure source. The handle portion includes a valve assembly with two trigger-operated valves. Activation of one of the valves establishes fluid communication between the vacuum source and the pipet-supporting portion, thereby causing liquid to be drawn into a pipet connected thereto. Activation of the other valve establishes fluid communication between the air pressure source and the pipet, thereby causing liquid contained within the pipet to be expelled therefrom. The liquid flow rate through the pipet is controlled by limiting the extent to which the trigger on the gun is depressed. In this manner, the operator may rapidly fill or void a major portion of the pipet by depressing the trigger fully or may slowly meter the pipet by depressing the trigger slightly.
In the pipet gun disclosed in the U.S. Pat. No. 3,963,061, the full range of liquid flow rates is achieved over the action path or depression stroke of the trigger. Due to the wide range of liquid flow rates and the limited action path of the trigger, however, slight movements of the trigger in such pipet guns produce significant changes in the liquid flow rate, which may result in ineffective pipetting procedures. Further, there is no indication for the operator to determine when the flow rate changes beyond a desired value.
U.S. Pat. No. 5,294,405, incorporated herein by reference, discloses an improved pipet gun having a wide range of liquid flow rates and avoids the "piston effect" of earlier devices by including adjustable valves for controlling the flow of liquid. In the pipet gun disclosed in the U.S. Pat. No. 5,294,405, the valves which control the air pressure and thus control the flow of liquid into and out of the pipet are adjustable between a plurality of flow rates. The valves slide longitudinally within a valve chamber. The valves operate by closing off the valve chamber from venting to ambient air, thus forcing air pressure which is directed into the valve chamber from an air pressure source into the pipet. When the adjustable valve is set on any one of the flow rates, for example slow, medium or fast, the pipet can be filled or emptied at that general flow rate. Further, the flow rate of the gun can be progressively increased over the path of the trigger by including tapering grooves along the valve body, which permit a graduation in the amount of air flow directed to the pipet. Such pipet guns, however, while extremely useful in pipetting fluids at a variety of flow rates, cannot effectively be adjusted from one general flow rate to another during expulsion of the liquid from the pipet, due to the risk of leakage and thus, can result in inaccurate pipetting. Further, while the valve attempts to close off the chamber from venting to ambient air over the entire path of the trigger, an effective airtight seal is not established, since the valve and valve chamber are typically constructed of plastic materials, thus decreasing the efficiency of the pipet gun. Additionally, while progressively increasing flow rates over the path of the trigger are useful, there is no indication for the operator to determine when the flow rate increases beyond a desired flow rate threshold.
While pipet guns having a wide range of flow rates utilizing only one flow rate at any particular trigger setting are useful, it is oftentimes desirable to significantly increase the flow rate of the liquid, specifically during expulsion of the liquid, for instance, to dislodge or "wash down" cells or particles attached to the side walls of a container while pipetting into the container.
Thus, it is apparent that a need exists for a pipet device which is capable of pipetting liquids at a variety of first flow rates, which is capable of adjustment during pipetting to a second flow rate which is in excess of the first flow rate and which is capable of distinguishing when a threshold is reached between a first flow rate and a second flow rate in excess of the first flow rate, without the problems associated with prior variable speed pipet devices, such as progressive changes in flow rate over the length of activation of the valve means.